The power amplifier (PA) is an important component part in an electronic device, as it can amplify the power of a weak electric signal to satisfy the need of transmission and emission. The energy for amplification comes from a DC power source, i.e. the PA converts DC energy into an AC signal to enable the power intensity of the AC signal to satisfy the demand. The capacity for the PA to convert the DC energy into AC energy is referred to as efficiency of the PA. The input/output signal power relation of the PA can be divided into a linear region, a nonlinear region and a saturated region.
When the envelope of an input signal fluctuates only in the linear region, the input signal is ideally amplified. But when the envelope of the input signal fluctuates to the nonlinear region, the output signal will be distorted. Such distortion manifests itself in the time domain as that the output signal is not an ideal amplification of the input signal, while manifests itself in the frequency domain as that the side lobe of the spectrum of the output signal is raised, and the main lobe is distorted.
Due to physical causes, when the envelope of the input signal fluctuates deep into the nonlinear region, efficiency of the PA is by far greater than the circumstance in which the envelope of the input signal fluctuates only in the linear region. With the advent of new modulation schemes, dynamic range of the signal envelope becomes larger and larger, so that the occurrence of nonlinear distortion is inevitable.
Baseband digital predistortion technique is an effective means in overcoming PA nonlinearity. By simulating inverse characteristics of the PA, the technique predistorts the baseband digital signal, so as to obtain an ideally amplified signal at the output terminal of the PA.
To simulate inverse characteristics of the PA in real-time, the traditional method needs to compare the input signal with the output feedback signal of the PA one sample point by one sample point, and this is referred to as vector method. The vector method is relatively high in cost because it demands precise feedback signal and demands precise synchronization of the input signal with the feedback signal. The scalar method as recently coming in vogue investigates and compares statistical characteristics of the feedback signal, so that the above problems are avoided.
FIG. 1 schematically illustrates the structure of a scalar method based baseband digital predistortion device. As shown in FIG. 1, the original signal x(n) first enters the predistorter 100 to be formed of a predistorted signal z(n), which is thereafter sent into power amplification 103 after passing through A/D converter 108 and up converter 109. The power amplified output signal y(n) is fed back from a coupler 104, is attenuated via an attenuator 105, and then enters a cost function generator 106. The cost function generated thereby is sent into a control update unit 107 that controls the operation of the predistorter 100. The cost function generator 106 and the control update unit 107 make up of a predistortion controller 110. The predistorter 100 mainly includes a predistortion data generation unit 101 and a multiplier 102. The predistortion data generation unit 101 generates predistortion data in accordance with the original input signal x(n) and a parameter provided by the control update unit 107, and the predistortion data is multiplied with the original signal by the multiplier 102 to obtain the predistorted signal z(n).
In the conventional predistorter, the cost function generator 106 calculates out-band power, and the control update unit 107 updates the predistortion parameter in accordance with the out-band power.
FIG. 2 is a view schematically illustrating a traditional predistorter 200. As shown in FIG. 2, the original signal x(n) is divided into three branches, one branch enters an amplitude predistortion data generation unit 201, and another branch enters a phase predistortion data generation unit 202 to obtain amplitude predistortion data and phase predistortion data respectively, while the remaining branch is multiplied with the amplitude predistortion data and the phase predistortion data respectively at multipliers 203 and 204 to obtain the predistorted signal z(n).
As the inventors of the present invention found during the development of the present invention, with the gradual increase in bandwidth of the communication system, frequency selectivity (memory effect) of the PA becomes gradually apparent. However, the traditional scalar method does not take memory effect into consideration. As a result, the performance of predistorter employing the conventional predistortion technique is far from being ideal.